Optical telecommunications utilize optical guides, called optical fibers because of their fiber-like structure, which can be used to create networks of varying degrees of complexity by connecting a plurality of user terminals.
If all network terminals are connected to a common central node, the network is called a star network.
The central node can be either active (signal repeater or switching network) or passive (optical signal coupler). In either case, the network's terminals can exchange numerical information in the form of coded pulse sequences, in serial fashion, with pulses of any sequence being transmitted one after the other in packets, in accordance with a predetermined access protocol.
Desirable requirements for a network access protocol to be used by terminals operating in real-time are:
(a) efficiency independent of the transmission speed of the coded pulse packets; PA1 (b) efficiency independent of both static transmission characteristics and traffic distribution; PA1 (d) ability to tolerate different distributions of such delay for different user classes;
(c) distribution of packet transmission delays within a small value range;
(e) provision for communication with other networks.
In accordance with the current state of the art, the above requirements may be met with network access protocols based on possible conflict prevention techniques, using an optical fiber star network.
A first typical example is the CSMA-CD (Carrier Sense Multiple Access with Collision Detection) technique. With this technique, a terminal sends a message immediately after having ascertained that the channel is clear. To this end, the received signal level is continuously monitored in order to detect possible collisions among simultaneously transmitted messages, since, if that has occurred, there will be a detectable change in the received signal level. After a collision is detected, each of the terminals involved stops any transmission then in process and, after the channel is clear, it commences retransmission in a way that is not likely to cause as second collision, for example, only after a random period of time has elapsed.
With an access protocol of this type, the greatest efficiency is achieved if the individual transmission intervals, and therefore the individual code pulse packets, are relatively long and if the signal propagation time among terminal pairs of the same network (propagation delay) is relatively short. This is inconsistent with very high transmission speeds since the packet transmission time becomes shorter and shorter with respect to signal propagation time.
An alternate conflict prevention technique for network access protocols will now be described which overcomes this latter difficulty. In this alternate technique each terminal transmits its own queue state data, so that each terminal has access to information regarding the position of all network users in a network-wide delay queue. This allows the automatic allocation of message transmission times for each terminal in accordance with a network delay queue collision avoidance algorithm, ensuring an optimal channel utilization efficiency which is independent of distribution and traffic characteristics.
That solution, however, has the drawback of requiring a terminal queue state data transmission protocol which itself is subject to the very disadvantages which it aims to eliminate--inefficient use of transmission capacity due to propagation time effects.